While Rosa has weakened to a tropical depression, the ongoing hazard of rainfall has put more than 11 million people under a flash flood warning.
According to the National Hurricane Centre, the rainfall may produce life-threatening flash flooding.
The heaviest rainfall is due to spread northward across northern Mexico and the desert southwest.
Dangerous debris flows and landslides are also possible in mountainous terrain, said the NHC.
Rosa is packing maximum sustained winds of 35mph, and is moving north at a speed of 10mph.
Mexico has discontinued its coastal warnings but is urging those in the area to remain vigilant.
Hazards associated with Rosa
The NHC is forecasting three to six inches of rainfall along Baja California and northwestern Sonora, with up to 10 inches in isolated areas.
Central and Southern Arizona are expecting two to four inches, with six inches possible in the mountains of Central Arizona.
The rest of the Desert Southwest, Central Rockies, and Great Basin will see one to two inches, four inches in isolated areas.
Tropical-storm-force wind gusts are also possible over the central portions of the Baja California peninsula, especially over higher elevations.
And surf will be a hazard too.
The NHC said swells generated by Rosa will affect portions of the coasts of southwestern Mexico, most of the west coast of the Baja California peninsula, and southern California on Tuesday.
“These swells are likely to cause life-threatening surf and rip current conditions,” said the NHC advisory.
Rosa, which was downgraded early Tuesday from a tropical storm, has already pounded the Northern Baja and Southern California desert regions.
The Mexican state of Sonora just south of the Arizona border has also taken a beating from the storm.
Where is Sergio headed?
While Sergio is a much stronger storm than Rosa, it looks set to head out to sea and shouldn’t pose a threat to land.
The storm is packing maximum sustained winds of 85 mpg.
Sergio is located about 840 miles southwest of the southern tip of Baja California, but is moving away in a westerly director at 14 mph.
Additional strengthening is expected during the next couple of days, and Sergio is forecast to be near major hurricane strength by Wednesday or Wednesday night, said the NHC.